The present invention relates generally to electrical ribbon cables and more particularly to electrical ribbon cables having twisted wire pairs.
The advantages of flexible ribbon cable are well known in the field. One of the principal advantages of electrical ribbon cable is the ability to mass terminate the cable using standard insulation displacement connectors known in the industry as Mass Termination Insulation Displacement (MTID) connectors. A plurality of parallel wires are bonded with a common insulator in a planar arrangement. There is a known, fixed distance, usually a uniform distance, between each conductor. The common insulation usually conforms somewhat to the conductors on at least one side of the cable to allow for ease in locating the conductors during termination. It is very easy and quick to mass terminate such an electrical ribbon cable. The cable is flat and flexible so that it takes up a minimum of space and can be easily positioned where desired between electrical equipments. Another advantage is that a large number of conductors can be located in a small amount of space. A still further advantage is a much neater appearance than would result from the same number of separate conductors.
A problem with electrical ribbon cable is a limitation in the electrical characteristics provided. The electrical characteristics are dictated by the long parallel runs of closely spaced conductors. Typically, these electrical ribbon cables have rather poor crosstalk characteristics and rather poor magnetic field immunity.
Twisted pair electrical cable has long been known to possess much better electrical characteristics than parallel lay wires or cable. A pair of insulated conductors are typically twisted together along the length of the cable. The geometry of the twisted pairs is known to produce the desirable electrical characteristics of low crosstalk and high immunity from interference from external magnetic fields.
However, the fundamental disadvantage of twisted pair cable is the difficulty of termination. In general, each conductor has to be individually terminated.
One solution has been to make an electrical ribbon cable with twisted wire pairs formed into an insulator in a planar ("ribbon") relationship. Periodically, e.g. every 20 inches (50.8 centimeters), along the cable the twisted wire pairs are run for a straight parallel segment, e.g. 2 inches (5.08 centimeters) in length, to allow for mass termination. In particular, see U.S. Pat. No. 4,034,148, Lang, Twisted Pair Multi-Conductor Ribbon Cable With Intermittent Straight Sections and U.S. Pat. No. 4,202,722, Paquin, Apparatus For Making Twisted Pair Multi-Conductor Ribbon Cable With Intermittent Straight Sections. The cable described is the so-called "Twist'N'Flat" cable. However, the straight ("flat") sections have significantly compromised the electrical characteristics of the cable while the twisted sections have significantly compromised the connectability characteristics of the cable. To minimize the deleterious effect on the electrical characteristics of the straight sections, these straight sections are spaced farther apart. However, this increased spacing means that the cable can be mass terminated only at these spaced straight sections, often 20 inches (50.8 centimeters) apart.
A ribbon cable disclosed in U.S. Pat. No. 3,736,366, Wittenberg, Mass Bonding of Twisted Pair Cables, is a ribbon cable that has a plurality of wire pairs having twist rates that are multiples of one another. At periodic distances along the cable, the various angular locations of the varying twists match. The cable then can be mass terminated using a "knife" to sever the surrounding insulation in order to allow the cable to be pulled apart to "create" a section where the conductors are parallel for mass termination. A disadvantage of this cable is that a "knife", i.e. a separate tool, must be used to modify the cable (sever the insulation material bonding the twisted wire pair conductors together) so that it again can be modified (stretched) to enable mass termination. A further disadvantage is that this opportunity for modification only occurs periodically along the cable which period is a multiple of all of the varying twist lengths.